The present invention relates to a system for sensing stress or damage in structural joints using fiber optic sensors.
The transmissivity of an optical fiber is maximized when the fiber is perfectly straight. Light is propagated through the fiber core by total internal reflection from the core/cladding boundary. When an optical fiber is bent, a portion of the light is refracted into the cladding and lost. The losses attributable to bending of the fiber measurably attenuate an optical signal transmitted from one end of the fiber to the other. This attenuation increases with an increase in the curvature of the bend. Sensitive photodetectors can detect minute changes in the intensity of light received from an optical fiber.
Practitioners in the art have developed fiber optic strain sensors that introduce light at one end of an optical fiber and detect changes in light intensity at the other end. The intensity varies in response to changes in curvature of the fiber caused by movement of the structure to which the fiber is attached. Mathematical relationships that relate the intensity change to the strain on the structure are known in the art.
Strain and movement can not only be detected but the location of the movement can be determined. U.S. Pat. No. 4,654,520 issued to Griffiths discloses a fiber optic system for continuously monitoring structural movement or stress. In the Griffiths system, a length of optical fiber is placed along a structure such as a pipeline. An optical signal is injected into one end of the pipeline. A detector at the same end is used to receive a reflection of the signal. The time period between injection of the signal and detection of its reflection can be used to determine the location of the anomaly along the fiber.
U.S. Pat. No. 4,692,610 issued to Szuchy discloses a system for monitoring structural strain. Szuchy mounts curved or serpentine sections of optical fiber on a flat surface of the structure. An optical time-domain reflectometer injects light pulses into the fiber and receives reflections. Variations in intensity of received light indicate strain on the structure. The fiber optic sensors of Szuchy have a sharp curvature to enhance detection of strain.
Other fiber optic sensor systems have been developed for detecting and locating structural damage. U.S. Pat. No. 4,581,527 issued to Crane et al. discloses a grid of optical fibers embedded in a plastic composite structure. A detection of light interruption or severe attenuation indicates damaged or highly stressed fibers.
The most highly stressed areas of aircraft structures are the structural joints. Joints may be bonded using a resin adhesive or may be fastened using a suitable fastener. Bonded joints between composite parts are especially susceptible to undetected damage, including cracking of the adhesive. These joints are often located in sealed, uninspectable areas of an aircraft structure. Critical joints may sustain damage that is uninspectable in normal use. Current practice requires that a structure suspected of having a failed joint be removed from service pending evaluation at a maintenance facility.
A fiber optic sensor can be incorporated into a joint by placing it between the structural members joined. Changes in reflections of an injected optical signal would indicate a discontinuity in the fiber caused by strain or damage to the joint. Practitioners in the art have not incorporated fiber optic strain monitoring systems into individual structural joints. Such a system would provide an economical and highly reliable method of monitoring these critical and often uninspectable areas without removing the aircraft from service. Theses problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.